Synthesis of ferrocene-modified carbon nitride photocatalysts by surface amidation reaction for phenol synthesis

Article abstract of DOI:10.1002/cjoc.201400229

A polymeric Fc-CO-NH-C₃N₄ (Fc-CN) material was synthesized by amidation reaction of ferrocenecarboxylic acid (Fc-COOH) with NH₂ groups on the surface of mesoporous graphitic carbon nitride (MCN). The properties of the as-synthesized samples were characterized by X-ray diffraction, Fourier transform infrared spectroscopy, UV-Vis diffuse reflectance spectra, N₂ adsorption-desorption isotherm, photoluminescence spectroscopy, transmission electron microscopy, electron paramagnetic resonance, (photo)electrochemical measurement and X-ray photoelectron spectroscopy. The resultant catalysts were investigated as heterogeneous catalysts for the selective oxidation of benzene to phenol using H₂O₂ as a green oxidant under visible light irradiation. The results reveal that Fc-modified samples can not only extend the visible light absorption, but also accelerate the bulk-to-surface charge transfer and separation via surface dyadic structures, both of which are favorable for phenol production from benzene photocatalytic hydroxylation with H₂O₂. Under the optimal conditions, up to 16.9 % phenol yield (based on benzene) is obtained by Fc-CN_/1.5-5 sample, and the corresponding Fe content is about 0.64 wt%. Furthermore, after the second run, no significant decrease of the activity (in term of TOF) and the selectivity is found in Fc-CN_/1.0-5 sample. Combined with the experimental results and Fenton-chemistry, a possible photocatalytic reaction mechanism on the hydroxylation of benzene to phenol at neutral medium with visible light is proposed. Copyright

Full text of DOI:10.1002/cjoc.201400229

FULL PAPER
DOI: 10.1002/cjoc.201400229
Synthesis of Ferrocene-Modified Carbon Nitride Photocata-
lysts by Surface Amidation Reaction for Phenol Synthesis
Xiangju Ye, Yun Zheng, and Xinchen Wang*
State Key Laboratory of Photocatalysis on Energy and Environment, College of Chemistry,
Fuzhou University, Fuzhou, Fujian 350002, China
A polymeric Fc-CO-NH-C3N4 (Fc-CN) material was synthesized by amidation reaction of ferrocenecarboxylic
acid (Fc-COOH) with NH2 groups on the surface of mesoporous graphitic carbon nitride (MCN). The properties of
the as-synthesized samples were characterized by X-ray diffraction, Fourier transform infrared spectroscopy,
UV-Vis diffuse reflectance spectra, N2 adsorption-desorption isotherm, photoluminescence spectroscopy, transmis-
sion electron microscopy, electron paramagnetic resonance, (photo)electrochemical measurement and X-ray photo-
electron spectroscopy. The resultant catalysts were investigated as heterogeneous catalysts for the selective oxida-
tion of benzene to phenol using H2O2 as a green oxidant under visible light irradiation. The results reveal that
Fc-modified samples can not only extend the visible light absorption, but also accelerate the bulk-to-surface charge
transfer and separation via surface dyadic structures, both of which are favorable for phenol production from ben-
zene photocatalytic hydroxylation with H2O2. Under the optimal conditions, up to 16.9 % phenol yield (based on
benzene) is obtained by Fc-CN/1.5-5 sample, and the corresponding Fe content is about 0.64 wt%. Furthermore, after
the second run, no significant decrease of the activity (in term of TOF) and the selectivity is found in Fc-CN/1.0-5
sample. Combined with the experimental results and Fenton-chemistry, a possible photocatalytic reaction mecha-
nism on the hydroxylation of benzene to phenol at neutral medium with visible light is proposed.
Keywords carbon nitride, ferrocenecarboxylic acid, benzene oxidation, photocatalysis, phenol
Introduction
however, it is a homogeneous system and depends
strongly on the presence of acid (H2SO4, HClO4,
CF3COOH), resulting in the difficulty in the catalyst
recovery and the corrosion of the reaction apparatus.[11]
To satisfy its industrial applications, many strategies
have been developed by introducing iron based hetero-
geneous catalysts, such as, carbon material or porous
silica supported iron-based materials.[12] Furthermore, to
avoid the over-oxidation of phenol, biphasic systems of
phase-transfer catalysis process have been devel-
oped.[11a]
Because sunlight is a clean, sustainable, and eco-
nomical energy source, at present, the introduction of a
promising process for the one-step conversion of ben-
zene to phenol based on the utilization of solar energy
under moderate conditions constitutes an area of per-
manent interest.[13] Photocatalysts such as TiO2 and
DDQ have been reported for benzene oxidation with
good photocatalytic activity.[14,15]
Phenol is an important raw material in the chemical
industry, which can be used to synthesize many impor-
tant chemicals.[1] Currently, the worldwide production
of phenol is mostly achieved by the three-step cumene
method. Unfortunately, this process produced a large
quantity of less-value acetone as byproduct.[2] To oper-
ate more economically and increase demand for phenol,
the development of a one-step oxygenation of benzene
to phenol has aroused widespread concerns of research-
ers. So far, considerable efforts have been devoted to
phenol production by direct hydroxylation of benzene,
including electrochemical and catalytic oxidation of
benzene.[3,4] O2, air/CO, N2O, H2O2, etc. have been used
as terminal oxidants for phenol production from ben-
zene.[5-8] However, they are typically operated under
high temperature (300－500 ℃) in gas-phase reaction
systems, leading to rapid deactivation of catalysts on
account of the coke formation during the reaction proc-
esses.[9] The liquid-phase oxidation reactions of benzene
in the presence of H2O2 have been reported to proceed
over iron-based catalysts due to their high catalytic per-
Recently, graphitic carbon nitride (CN), a two-di-
mensional π-conjugated polymeric semiconductor, has
been demonstrated to exhibit outstanding photocatalytic
performance in several important fields.[16-19] It is worth
noting that MCN could catalyze one-step oxidation of
benzene to phenol with H2O2 under visible light irradia-
formance.[10] Among them, the Fenton reagents
＋
(Fe2 -H2O2) have been developed for many decades;
*
E-mail: xcwang@fzu.edu.cn; Tel.: 0086-591-83920097; Fax: 0086-591-83920097
Received April 14, 2014; accepted May 7, 2014; published online May 22, 2014.
498
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Chin. J. Chem. 2014, 32, 498—506

Synthesis of Ferrocene-Modified Carbon Nitride Photocatalysts by Surface Amidation Reaction
tion. Moreover, when Fe-doped carbon nitride (Fe-CN)
is deposited into the titanium silicate zeolite (TS-1), the
activity of this material is improved obviously. However,
the stability of the catalyst is reduced owing to the lack
of strong chemical interaction between Fe-CN guest and
TS-1 host.[20]
cenecarboxylic acid (Aladdin), ammonium bifluoride,
acetonitrile, benzene, toluene, N,N-dimethylformamide,
dichloromethane, ethanol, hydrogen peroxide (30%,
w/w), triethylamine were purchased from Sinopharm
Chemical Reagent Co., Ltd. (Shanghai, China). All of
the materials were used as received without further pu-
rification.
Experimentally, due to the imperfect condensation of
the precursor, there is a high density of NH2 groups as
the residual moiety on the surface of the polymeric car-
bon nitride material.[21] Several protocols based on
grafting reaction have been devised for engineering of
carbon nitride polymeric materials.[22] Among them,
surface organometallic functionalization has been proved
to be a favorable route for the generation of highly reac-
tive moiety by introducing donor-acceptor heterogene-
ous structure. The commonly delocalized π system
could improve the ability of the material to harvest the
sunlight and the migration of photogenerated carriers,
modulating the (photo) electric properties of polymeric
semiconductors at a molecular level. Most recently, we
have synthesized a photocatalyst by the Schiff base re-
action between the ferrocene carboxyaldehyde (Fc-CHO)
and the NH2 group of the MCN, and it can be used to
catalyze the direct phenol production from benzene with
H2O2 and visible light under mild conditions.[23]
In this work, we developed an alternative strategy by
amidation reaction to construct a new polymeric CN
material by reaction of Fc-COOH with NH2 groups of
MCN. Ferrocenoyl chloride (Fc-COCl) and MCN were
synthesized by the previous methods.[24,25] Then, a lone
pair of electrons of amino groups at the end of the car-
bon nitride polymer nucleophilically attacks on the car-
bon center of the formed Fc-COCl in anhydrous di-
chloromethane, and then loses a molecule of hydrogen
chloride (the detail in Experimental Section). Organo-
metallic active species such as ferrocene moieties are
thus immobilized onto the CN framework via an amide
bond (CO-NH). The formed surface dyadic structures,
in principle, not only extend the absorption of visible
light, but also stabilize the hybrid system, and accelerate
the bulk-to-surface charge transfer and separation. The
reaction is depicted in Scheme 1. A series of MCNr (r
refers to the initial silica/cyanamide mass ratio, Experi-
mental Section) were synthesized by the nanocasting
approach using SiO2 nanoparticles as the template.[25]
The obtained samples are denoted as Fc-CNr-X, where X
is an arbitrary number that represents the initial concen-
tration of Fc-COCl. The as-obtained samples were sub-
jected to several characterizations and its photocatalytic
activities for hydroxylation of benzene to phenol using
H2O2 were evaluated in a biphasic H2O-H2O2/aceto-
nitrile medium under visible light irradiation.
Preparation
MCN/r (r refers to the initial silica/cyanamide mass
ratio) was prepared according to the reported proce-
dure.[25] In a typical synthesis, 3 g of cyanamide was
dissolved in different amounts of a 40% dispersion of
12 nm SiO2 particles (Ludox HS40) in water (1.5, 7.5,
11.25 g Ludox for the r＝0.2, 1, 1.5) with stirring in an
oil bath at 373 K until removal of water and formation
of a white solid. The powder was grounded in a mortar,
transferred into a crucible and heated under air at 2.3
K/min (4 h) up to 823 K and then held at this tempera-
ture for 4 h. The as-obtained yellow powders were
treated with 4 mol/L ammonium bifluoride for 2 d in
order to remove the silica templates, and then the pre-
cipitates were washed with deionized water and ethanol
several times. After filtering, the yellow compounds
were dried under vacuum at 353 K overnight. To pre-
pare bulk graphitic CN (abbreviated as CN/0), cyana-
mide was directly heated to 823 K and then held at this
temperature for 4 h.
Fc-CNr-X: Fc-CNr-X was synthesized by amidation
reaction. In a typical procedure, Fc-COOH (1.15 g, 5
mmol) was dissolved in 30 mL anhydrous dichloro-
methane in an ice bath and stirred under N2 atmosphere.
A solution with oxalyl chloride (0.56 mL, 6.5 mmol)
and 0.1 mL N,N-dimethylformamide was added under
vigorous stirring for 0.5 h. After stirring for another 2 h,
the residue (Fc-COCl) was collected under vacuum to
remove the solvent. Subsequently, 0.5 g MCNr was sus-
pended in a 60 mL anhydrous dichloromethane in an ice
bath under N2 atmosphere. The solution with different
concentrations of as-prepared Fc-COCl (CFcCOCl＝0.67,
1.67, 3.35, 6.7, 16.7 mmol/L for the X＝1, 2, 3, 4, 5,
respectively) was added under vigorous stirring in the
presence of triethylamine for 0.5 h. After stirring for
another 10 h, a grey solid was collected under vacuum
to remove the solvent. The precipitate was washed with
anhydrous dichloromethane for several times to remove
un-reacted Fc-COCl and dried at 333 K for 12 h.
Characterization
Powder X-ray diffraction (XRD) patterns of the
samples were collected by using a Bruker D8 Advance
X-ray diffractometer (Cu Kα1 irradiation, λ＝1.5406 Å).
The optical properties of the samples were characterized
by a Varian Cary 500 Scan UV-Vis diffuse reflectance
spectrophotometer (DRS), and BaSO4 was employed as
the internal reflectance standard. Fourier transformed
infrared (FT-IR) spectra were achieved by using a
Nicolet Magna 670 FTIR spectrometer and the samples
Experimental
Materials
Cyanamide (Alfa Aesar), Ludox HS40 (40 wt% SiO2
in water, Aldrich), oxalyl chloride (Aladdin), ferro-
Chin. J. Chem. 2014, 32, 498—506
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Ye, Zheng & Wang
FULL PAPER
Scheme 1 A scheme of the synthesis of Fc-CN
(COCl)2
O
O
C
O
C
Fe
Fe
Fe
CH2Cl2
C
OH
Cl
NH
N
N
N
N
N
N
NH2
N
CH2Cl2
Et3N
N
N
HN
N
N
NH
N
N
N
N
N
N
N
N
N
N
550 oC, 4 h
HN
N
N
N
NH
N
N
N
NH2
N
N
NH2
N
N
N
NH2
N
N
H
N
N
N
N
N
N
NH2
N
N
N
N
NH2
N
Fc-CN
H
MCN
were mixed with KBr at a concentration of ca. 1 wt%.
N2 adsorption-desorption analyses were carried out at
77 K using Micromeritics ASAP 2010 Surface Area and
Porosity Analyzer. The Fe content data for the samples
were recorded using Inductively Coupled Plasma Opti-
cal Emission Spectrometer (ICP-OES) (Ultima 2, Jobin
Yvon). Photoluminescence (PL) spectra of the samples
were recorded on an Edinburgh FI/ FSTCSPC 920
spectrophotometer. The excitation wavelength was set at
400 nm. X-ray photoelectron spectroscopy (XPS)
tion to get a slurry mixture. The slurry was spread onto
ITO glass whose side part was previously protected by a
Scotch tape. The working electrode was dried overnight
under ambient conditions. Uncoated parts of the elec-
trode were isolated with epoxy resin. Pt and Ag/AgCl (3
mol/L KCl) electrodes were used as a reference and a
counter electrode, respectively. A 300 W Xe lamp to-
gether with a visible light band-pass filter (＞420 nm)
was applied as the excitation light source.
Photocatalytic test
measurements were performed on
a
Thermo
To evaluate the photocatalytic activity of the result-
ing samples, the catalytic oxygenation tests of benzene
to phenol by Fc-CN were carried out according to the
reported procedure.[13] In a typical oxidation system,
Fc-CN (50 mg) was suspended in a mixture of acetoni-
trile (4 mL), benzene (0.8 mL, 9 mmol), water (4 mL),
and hydrogen peroxide (0.51 mL, 5 mmol). The result-
ing biphasic system was stirred at 333 K for 4 h under
visible light irradiation together with a 420 nm cutoff
filter (300 W Xenon lamp). At the end of the reaction,
ethanol (5 mL) was poured into the mixture at 277 K to
quench the reaction and to turn the biphasic system to
single-phase one. As a comparison, the reaction system
was performed under the same conditions except with-
out visible light irradiation. The products of the reac-
tions were analyzed using a high performance liquid
chromatograph (HPLC2487, C18, 4.6 mm×250 mm,
5-Micron) with toluene as the internal standard. H2O2
concentration was determined by a colorimetric titration
method based on the formation of a yellow colored
complex TiIV-H2O2, using a UV-Vis spectrophotometer
at 410 nm.
ESCALAB250 spectrometer, which consists of a
monochromatic Al Kα as the X-ray source. For obtain-
ing TEM images, the as-obtained samples were exam-
ined using a JEOL model JEM 2010 EX instrument at
an accelerating voltage of 200 kV.
Electron Paramagnetic Resonance (EPR) measure-
ments were operated at a Bruker EPR A300 spectrome-
ter. Typically, a 10 mg portion of Fc-CN/1.0-5 powder
was placed in a quartz-glass sample tube. The settings
for the ESR spectrometer were: center field: 3512.54 G;
modulation frequency: 100.00 KHz; microwave fre-
quency: 9.86 GHz; power: 2.00 mW. For the pure and
Fc-modified MCN catalysts, ESR measurements were
carried out under dark condition. For clarifying the ef-
fect of light-induced process, the ESR analyses of MCN
and Fc-CN were operated under visible light irradiation.
A 300 W Xenon lamp (PLS-SXE 300, Beijing Perfect
Light Co. Ltd.) was employed as the light source with a
cutoff filter (＞420 nm). A light guide was used to di-
rect the irradiation onto the powder sample through the
quartz window of the cell for experiments.
Electrochemical measurements were conducted in a
BAS Epsilon Electrochemical System under visible
light irradiation with a conventional three electrode cell,
immersing in the solution of 0.1 mol/L Na2SO4 (50 mL,
pH 6.8). The Fc-CN electrodes were employed as the
working electrodes, which were prepared on indium-tin
oxide (ITO) conductor glasses. The 10 mg sample was
dispersed in 800 µL N,N-dimethylformamide by sonica-
Results and Discussion
Physicochemical properties of Fc-CN
To investigate the structure, texture, optical and
electronic properties of the as-synthesized samples, a
series of characterizations (such as XRD, FT-IR, DRS,
500
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Synthesis of Ferrocene-Modified Carbon Nitride Photocatalysts by Surface Amidation Reaction
PL, EPR and so on) were performed.
polymer.
N2 adsorption-desorption measurements were used
As shown in Figure 1, the X-ray powder diffraction
(XRD) patterns of Fc-CN/1.0 and MCN/1.0 are nearly
identical, exhibiting the typical graphitic interlayer (002)
peak at 27.4° (corresponding to 0.326 nm) and in-plane
structural packing motif (100) peak at 13.0° (corre-
sponding to 0.681 nm). The absence of XRD peaks
originating from iron species is observed after Fc moi-
ety modification. It is worth mentioning that the intensi-
ties of (002) peaks for Fc-CN/1.0 samples decrease
gradually, which implies that the layered graphitic
structure is somewhat alternated since macromolecular
Fc moiety anchored into carbon nitride framework.
to identify the structure of the as-obtained samples. The
isotherms and the Barret-Joyner-Halenda (BJH) pore
size distributions of the materials are displayed in Fig-
ure 3. They possess type IV adsorption-desorption iso-
therms and H1 hysteresis loops, indicating the existence
of mesoporous structure in the MCN/1.0 and Fc-CN/1.0-5
samples. From experimental results, we can see that the
specific surface area and pore volume decrease from
187 m2/g and 0.50 cm3/g of the pristine MCN/1.0 to 67
m2/g and 0.21 cm3/g of the Fc-CN/1.0-5, respectively,
presumably due to that the macromolecular Fc moieties
block the hole of the host material.
0.004
350
0.003
MCN/1.0
300
0.002
250
200
150
100
50
0.001
0.000
.
10 20 30 40 50 60 70 80
Pore size/nm
MCN/1.0
Fc-CN/1.0-5
Fc-CN/1.0-5
10
20
30
40
50
60
0
2θ /( o )
0.0
0.2
0.4
0.6
0.8
1.0
p/po
Figure 1 XRD patterns for MCN/1.0 and Fc-CN/1.0 samples.
Figure 3 N2 adsorption-desorption isotherms of MCN/1.0 and
Fc-CN/1.0-5; inset is the corresponding BJH pore-size distribution
curves.
The FT-IR spectra of the as-synthesized samples are
shown in Figure 2. Similar to MCN/1.0, characteristic
bands of aromatic CN heterocycles at 1200－1600 cm−1
and heptazine units at 800 cm−1 are found in Fc-CN/1.0
.
Transmission electron microscope equipped with
energy dispersive X-ray analyzer (TEM-EDX) was in-
troduced to learn the texture of the sample. From Figure
4, the presence of large-scale mesoporous structure and
well-defined pore system is found in Fc-CN/1.0-5 sample,
signifying that this organic protocol does not alter the
pristine nanostructure or the core chemical skeleton of
MCN/1.0 matrix. Most importantly, the signal of EDX
analysis confirmed the presence of iron in Fc-CN/1.0-5
sample.
The vibration of the formed CO-NH group is hardly
observed in Fc-CN/1.0 samples, which is probably owing
to the fact that its vibration is overlapped by that of C－
N－H (1557 cm−1). However, it is noted that the broad
peak at around 3000－3500 cm−1 for the Fc-CN samples
corresponds to secondary and primary amines for the
incomplete condensation of the CN framework, where
the intensity decreases gradually with the increasing
amount of Fc moiety. It once again indicates that the Fc
moiety is successfully grafted into the carbon nitride
The optical features of the as-prepared Fc-CN/1.0
materials were examined by DRS. In Figure 5, it is ob-
vious that all samples have semiconductor absorption in
the visible light region due to the excitation of electron
from the valence band to the conduction band. As a re-
sult of the Fc-mediated function, the absorption edges of
Fc-CN/1.0 samples are gradually red-shifted compared to
that of MCN/1.0 host, due to the enlargement of
π-conjugated electron delocalization in the surface of
Fc-CN/1.0 with the increasing amount of Fc moiety. In
fact, the colors of the samples shift from yellowish to
grey with the increasing Fc content. These results indi-
cate that Fc entities are indeed incorporated into the
polymeric MCN frameworks.
Fc-CN/1.0-5
MCN/1.0
4000 3500 3000 2500 2000 1500 1000 500
ν /cm-1
Figure 2 FT-IR spectra of MCN/1.0 and Fc-CN/1.0
.
Chin. J. Chem. 2014, 32, 498—506
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501

Ye, Zheng & Wang
FULL PAPER
MCN/1.0
Fc-CN/1.0-5
450
500
550
600
650
700
λ/nm
Figure 6 PL spectra of MCN/1.0 and Fc-CN/1.0-5 sample.
decreased semicircles for Fc-CN/1.0-5 under visible light
irradiation compared with pure MCN/1.0, revealing an
obvious improvement in the electronic conductivity af-
ter the modification with Fc moiety (Figure 7a). Mean-
while, this result can also be supplemented by photo-
current tests (Figure 7b). A remarkable enhancement in
the photocurrent is observed for Fc-CN/1.0-5 sample,
which is a factor of 4.0 to that of MCN/1.0 host. The
Figure 4 TEM-EDX images of Fc-CN/1.0-5.
25
20
15
10
5
(a)
MCN/1.0
Fc-CN/1.0-5
0
0
10
20
30
40
50
Z/kΩ
Figure 5 Optical absorption of MCN/1.0 and Fc-CN/1.0 samples.
(b)
0
To investigate the efficiency of charge-carriers
separation/recombination of the materials, the room
temperature photoluminescence (PL) spectra for
MCN/1.0 and Fc-CN/1.0-5 samples were performed on the
excitation light of 400 nm (Figure 6). An apparent fluo-
rescence quenching is observed for Fc-CN/1.0-5 sample
compared to that of MCN/1.0 polymeric semiconductor,
suggesting that the radiative charge recombination has
been efficiently suppressed after the incorporation of Fc
in MCN matrix. This is attributed to the optimized elec-
tronic properties of the Fc-grafted polymeric material
that facilitates charge separation.
The photoelectrochemical properties of Fc-CN/1.0
under visible light irradiation were also examined using
a BAS Epsilon Electrochemical System with a conven-
tional three electrode cell. In the electrochemical im-
pedance spectroscopy (EIS), we can find a significantly
-1
-2
.
MCN/1.0
Fc-CN/1.0-5
on
40
off
0
20
60
80
100
t/s
Figure 7 Photoelectrochemical properties of MCN/1.0 and
Fc-CN/1.0-5 samples in a 0.1 mol/L Na2SO4 aqueous solution (pH
6.8) under visible light irradiation (λ＞420 nm). (a) EIS Nyquist
plots at −0.1 V bias potential vs. Ag/AgCl and (b) the periodic
on/off photocurrent response at −0.2 V bias potential vs.
Ag/AgCl.
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Synthesis of Ferrocene-Modified Carbon Nitride Photocatalysts by Surface Amidation Reaction
photoelectrochemical results signify that the Fc-func-
tional MCN/1.0 exhibited enhanced charge carries trans-
fer and separation.
tion by Fc-CN/1.0 photocatalysts is obtained. In addition,
the phenol yield gradually enhanced with the increasing
amount of the grafted Fc moiety, suggesting that iron
centers are considered as the dominant catalytic active
sites during the catalytic oxidation of benzene and the
photocatalytic performance (phenol yield and turnover
frequency) is closely correlated with the grafted content
of Fc moiety (Table 1, Entries 4－6, 8, 9). For example,
when using Fc-CN/1.0-1 sample, the phenol yield is
4.2% with a low TOF (TOF＝0.345 h−1). While with the
Fc group content increasing from 0.66 to 1.6 wt%, the
phenol yield is promoted from 4.2% to 13.7% and the
TOF is improved from 0.345 to 1.135 h−1. Simultane-
ously, the corresponding phenol selectivity (based on
H2O2) and H2O2 conversion also increased. The optimal
sample for the benzene conversion is Fc-CN/1.0-5, of
which the corresponding Fe content is 0.24 mg. (Entry 9
of Table 1). Both the benzene conversion and TOF ob-
viously improve by a factor of 19.6 over the parental
MCN/1.0 sample. For comparison, only a small amount
(2.5%) of phenol production is detected when Fc is
added as a catalyst under visible light illumination (Ta-
ble 1, Entry 1). In contrast with the dark reaction,
Fc-CN/1.0-4 exhibited distinct conversion rates under
visible light irradiation (Table 1, Entries 7, 8), indicat-
ing that this reaction is indeed a photocatalytic reaction
process. When a mixture (Fc/MCN/1.0) is used as cata-
lyst on the catalytic reaction, the phenol formation of
9.4% is achieved (Table 1, Entry 13). A control experi-
ment is carried out in the absence of a catalyst or an
oxidant (H2O2), the reaction does not occur at all (Table
1, Entries 11, 12,). Notably, some by-products (such as
catechol, hydroquinone and benzoquinone) are also de-
tected in the reaction products due to the imbalance be-
tween the decomposition rate of H2O2 and the oxidation
rate of benzene during the oxidation process, but the
total yield of them is low. More importantly, no biphe-
nyl by-product is detected in our system. This is in
agreement with our previous report.[23]
Electron paramagnetic resonance (EPR) was applied
to study the electronic band structure of the resulting
samples (Figure 8). One single Lorentzian line centering
at a g＝2.0034 is presented in CN material, which cor-
responds to an unpaired electron on carbon atoms of the
aromatic rings within the π-bonded nanosized clus-
ters.[26] It is interesting that the intensity of the EPR
signals of the materials enhances with the increasing
carbon amount in the dark. In contrast to MCN/1.0, the
EPR signal for Fc-CN/1.0-5 remarkably increases after
covalently grafting with Fc group. In addition, under
visible light irradiation, significantly enhanced EPR
signals of the samples are found, revealing the efficient
photochemical generation of radical pairs in the Fc-
based polymeric semiconductor.
Fc-CN/1.0-5
MCN/1.0
Dark
Light
3460
3480
3500
X/G
3520
3540
3560
Figure 8 Solid-state EPR spectra of MCN/1.0 and Fc-CN/1.0-5
sample.
The Fc-mediated carbon nitride material is efficient
in the activation of H2O2 under mild conditions, so the
photocatalytic tests on the selective oxidation of ben-
zene to phenol by Fc-CN samples were evaluated in the
sealed glass-reactor under visible light irradiation with a
biphasic H2O2-H2O/acetonitrile medium, in which the
synthetic phenol is extracted from water into organic
phase to minimize over-oxidation.
Effect of Fc-CNr-5 catalysts with different surface
area of the substrate on the conversion of benzene to
phenol
The surface area of the material is considered to be
one of the most important factors for catalysis/photo-
catalysis.[19] The activities of Fc-CN/r-5 samples with
different surface area of substrate towards the oxidation
of benzene to phenol are displayed in Table 2. Accord-
ing to the point of surface chemistry, the larger the sur-
face area of the MCN matrix is, the higher the density of
the exposed NH2 group becomes. This is favorable for
the grafting of the Fc moiety and then leads to an im-
provement in benzene conversion and TOF. For in-
stance, the phenol yield and TOF are 3.3% and 0.273
h−1 of Fc-CN/0-5 sample, which are lower than those of
Fc-CN/1.0-1 catalyst. The results clearly revealed that the
large surface area of the catalyst not only is beneficial
for light-harvesting and mass-transfer, but also
Effect of different content of Fc grafted MCN/1.0
photocatalysts on the hydroxylation of benzene to
phenol
In our previous study, we have demonstrated that
MCN/1.0 can be applied to the direct conversion of ben-
zene to phenol in the presence of H2O2 under visible
light irradiation, but the activity is low (phenol yield
based on benzene is 0.7% and TOF＝0.058 h−1). In or-
der to improve its photocatalytic activity, a series of
Fc-CN/1.0 polymeric materials with different Fc contents
are synthesized via amidation reactions and the catalytic
results are summarized in Table 1. After modifying with
Fc moiety, a significant improvement in phenol forma-
Chin. J. Chem. 2014, 32, 498—506
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Table 1 Liquid-phase oxidation of benzene to phenol by Fc-CN/1.0 catalysts with different Fc content with (＋)/without (－) visible light
(＞420 nm)a
OH
Fc-CN, H2O2
60 oC, Visible light
Yieldc/%
Entry
Catalyst
MCN/1.0
Fcb/mg
hν
H2O2 conv.d/%
PH sel.e/%
TOF f×100/h−1
PH
CA
HQ
BQ
1
2
—
＋
—
＋
＋
＋
＋
—
＋
＋
＋
＋
＋
＋
0.7 n.d.g n.d.
n.d.
n.d.
0.1
0.6
0.5
1.2
t.a.
1.1
1.2
n.d.
n.d.
n.d.
3.3
7.8
4.9
14.8
9.2
5.8
2.5
0.3
2.5
4.2
6.9
9.0
1.3
n.d. n.d.
Fc
2.0
3
t.a.h
t.a.
t.a.
0.3
t.a.
t.a.
t.a.
t.a.
t.a.
t.a.
t.a.
0.5
23.4
62.2
73.8
84.7
13.2
100
99.1
2.0
19.3
12.1
16.9
19.2
17.4
19.5
25.2
t.a.
20.7
34.5
57.2
74.6
10.7
92.7
113.5
t.a.
4
Fc-CN/1.0-1
Fc-CN/1.0-2
Fc-CN/1.0-3
0.33
0.43
0.60
5
6
7
Fc-CN/1.0-4
0.67
0.80
8
11.2 0.2
13.7 0.4
9
10i
Fc-CN/1.0-5
t.a.
n.d. n.d.
11
—
—
n.d. n.d. n.d.
n.d. n.d. n.d.
1.2
n.d.
n.d.
16.3
n.d.
12j Fc-CN/1.0-5
0.80
0.80
—
n.d.
13 Fc/MCN/1.0
9.4
0.3
0.1
100
77.9
b
c
a Reaction conditions, see Experimental Section. Determined by Inductively Coupled Plasma optical emission spectrometer. Moles of
d
product/moles of initial benzene×100. PH＝phenol, CA＝catechol, HQ＝hydroquinone, BQ＝benzoquinone. Moles of reacted H2O2/
moles of initial H2O2×100. e Moles of produced phenol/moles of reacted H2O2×100. f Turnover frequency: n(phenol) per n(melem units)
per hour. g Not detectable. h Trace amount. i 3 mL ethanol was added to the reaction system. j The reaction was performed under the same
conditions except without H2O2.
Table 2 Liquid-phase oxidation of benzene to phenol by a series of Fc-CNr-5 catalysts with visible light (＞420 nm) for 4 ha
Yield/%
SAb/(m2•g－
)
Fc/mg
H2O2 conv./%
PH sel./%
TOF×100/h−1
1
Entry
Catalyst
PH CA HQ BQ
1
2
3
4
Fc-CN/0-5
Fc-CN/0.2-5
Fc-CN/1.0-5
Fc-CN/1.5-5
10
55
0.27
0.50
0.80
1.07
3.3 0.1 t.a. 0.3
8.1 0.3 0.2 0.6
13.7 0.4 0.5 1.2
16.9 0.6 0.4 1.8
58.2
98.3
99.1
97.0
10.5
15.0
25.2
31.7
27.3
67.1
187
113.5
140.0
290
a Reaction conditions, see Experimental Section. b Surface area.
improves the chance of the amidation reaction between
Fc-COOH and MCN polymer, thus it greatly improves
photosynthesis of phenol from benzene with H2O2 under
visible light irradiation. Simultaneously, the corre-
sponding phenol selectivity also increases. Among all
the samples evaluated in this series, Fc-CN/1.5-5 showed
the highest photocatalytic reactivity in phenol produc-
tion. Importantly, the activity increased with increasing
Fc content, and this is proportional to the enhanced sur-
face area of the parental materials.
The stability of a catalyst plays an important role in
its assessment and industrial application. Thus, oxy-
genation reaction of benzene was carried out with a pro-
longed reaction time with visible light by Fc-CN/1.0-5
catalyst. We operated the oxidation reaction for 16 h,
and after every 4 h of the reaction, the used Fc-CN/1.0-5
catalyst was washed with acetonitrile and ethanol for
several times, dried at 80 ℃ overnight and then
re-employed in a new reactant mixture. The catalytic
activity of the sample for phenol production during the
four runs is displayed in Figure 9. The total phenol syn-
thesis from benzene across the four runs is 4.02 mmol
and the corresponding TOF with respect to the
tri-s-trizine (C6N8) repeating units is 3.71 h−1, which
once again confirms the photocatalytic nature of this
system. In the second run, the TOF somewhat decreases
from 1.135 to 0.86 h−1. However, in the subsequent con-
tinuous three runs, the catalyst maintains high TOF at
about 0.86 h−1, and no obvious deactivation is observed.
In addition, XRD pattern, FT-IR and XPS spectra of the
catalyst before and after the catalytic reaction again il-
lustrate the good stability of the Fc-CN/1.0-5 material
(Figure 10).
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Chin. J. Chem. 2014, 32, 498—506

Synthesis of Ferrocene-Modified Carbon Nitride Photocatalysts by Surface Amidation Reaction
120
(a)
Fc-CN/1.0-5
80
Fresh Fc-CN/1.0-5
Used Fc-CN/1.0-5
40
0
1st
2nd
3rd
4th
10
20
30
40
50
60
2θ /(o )
Figure 9 Cyclic runs in the photocatalytic oxidation of benzene
to phenol under visible-light irradiation (λ＞420 nm) by Fc-
CN/1.0-5.
(b)
Fresh Fc-CN/1.0-5
Combined with the above results and Fenton reac-
Ⅱ
Ⅲ
tion, a reversible Fc -CN/Fc -CN redox mechanistic
proposal for the catalytic reaction is given in Scheme 2.
The photocatalytic cycle starts as follows: hydroxyl
Ⅲ
Used Fc-CN/1.0-5
radicals (•OH) and Fc -CN are produced in the pres-
ence of Fc -CN, H2O2 and H＋ (from H2O), and the
Ⅱ
photogenerated electron-hole pairs are excited and
quickly transfer to the conduction band and valence
band of CN material under visible light irradiation, re-
spectively. Subsequently, the photo-generated electrons
in the conduction band is absorbed by the surface-
4000 3500 3000 2500 2000 1500 1000 500
ν /cm-1
Ⅲ
Ⅱ
bonded Fc -CN species to regenerate Fc -CN catalyst,
and the generated •OH attack the aromatic ring, fol-
lowed by the formation of the hydroxycyclohexadienyl
radicals.[27] Then, the positive holes of CN host (h＋)
with moderate oxidation potential oxidize hydroxycy-
clohexadienyl radicals to yield phenol, H＋ and e－, in
which the electrons offset the holes in the valence band
of MCN and the releasing H＋ returns the reaction sys-
tem to restart the catalytic cycle, respectively. In addi-
tion, when ethanol, a hydroxyl radical scavenger, was
added into the reaction system, a trace amount of phenol
was detected, indicating that the oxidation indeed pro-
ceed a hydroxyl radical-induced catalytic process. (En-
try 10, Table 1).
Fe 2p
707.9 eV
(c)
711.4 eV
714.7 eV
720.6 eV
725.1 eV
Fresh Fc-CN/1.0-5
Used Fc-CN)/1.0-5
730
725
720
715
710
705
BE/eV
Figure 10 (a) XRD pattern; (b) FT-IR and (c) XPS spectra of
Fc-CN/1.0-5 before and after the catalytic reaction.
Scheme 2 Proposed reaction mechanism for benzene hydroxy-
lation by Fc-CN
Conclusions
In conclusion, a new polymeric Fc-CN material has
been successfully synthesized by amidation reaction of
ferrocenecarboxylic acid (Fc-COOH) with aromatic
NH2 groups of polymeric MCN. With the synergistic
donor-acceptor interaction between the MCN matrix
and Fc motifs, photocatalytic performance towards the
phenol production from benzene in the presence of
H2O2 under visible light irradiation is achieved. Under
the optimal conditions, the highest photocatalytic activ-
ity of the sample among the Fc-CN/1.0 examined is Fc-
CN/1.0-5, which is 19.6 times higher than that of the pa-
rental MCN/1.0. This work demonstrates that Fc-CN
OH
e-
O
H
Fe
N
N
H2N
N
N
N
N
N
N
N
N
C
H2O2
N
H+
N
N
N
N
N
N
OH
H
OH
H
HN
NH
N
N
N
e-
h+
N
N
Fc-CN
Chin. J. Chem. 2014, 32, 498—506
© 2014 SIOC, CAS, Shanghai, & WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
www.cjc.wiley-vch.de
505

Ye, Zheng & Wang
FULL PAPER
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Acknowledgement
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This work is financially supported by the National
Basic Research Program of China (973 Program) (Grant
No. 2013CB632405), the National Natural Science
Foundation of China (Grant Nos. 21033003 and
21173043), the State Key Laboratory of NBC Protection
for Civilian (No. SKLNBC2013-04K), and the Special-
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